Rock bits for a drilling machine used for forming big holes in tunnels in rock and other formations are described in U.S. Pat. Nos. 3,389,760, 3,658,141 and 3,786,879. Such bits can have a rotary head with a plurality of cutter assemblies mounted on the rotary head. Each cutter assembly can comprise a yoke having a base for attachment to the rotary head and a pair of legs depending from the base. Each cutter can be mounted on a load pin extending between the legs of the yoke, with the ends of the load pin being disposed within a pair of aligned openings in the legs.
In this type of drill bit the load of drilling is transmitted to the yoke via the load pin. Because of this load, fretting of the load pin and the legs of the yoke around the periphery of the mounting holes occurs. Due to this fretting, each load pin is usually replaced every second or third time the cutter mounted on the load pin is replaced.
It is desirable to avoid fretting of the yoke because the mounting holes for the load pin can become out of round. When they become out of round, it is necessary to cut off the old yoke and weld a new one on the rotary head. This is a costly and time consuming process.
To avoid such wear of the yoke, a full wear bushing can be placed in each opening in the yoke legs as described in U.S. Pat. No. 3,389,760. Segmented bushings have also been used to prevent wear. However, because the non-loaded portions of each leg at the end of the legs are inherently thin so they do not scrape against formation during drilling, only thin bushings have been used in the mounting holes to avoid undue weakening of the unloaded portion of the legs. Because a thin bushing is used, breakage of the loaded portion of the bushing can occur. This leaves the loaded portion of the yoke unprotected from fretting with resultant premature wear of the yoke necessitating costly and time consuming replacement.
Therefore, there is a need for a roller cutter assembly for a drill bit where the load pin of the roller cutter assembly is mounted between the legs of the yoke in such a way that fretting of the legs is avoided during drilling.